netdev - Re: [PATCH v4] lpc32xx: Added ethernet driver: smp

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Message-ID: <20120306141745.GA15201@n2100.arm.linux.org.uk>
Date:	Tue, 6 Mar 2012 14:17:45 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	Roland Stigge <stigge@...com.de>, alexander.h.duyck@...el.com,
	baruch@...s.co.il, ian.campbell@...rix.com, arnd@...db.de,
	netdev@...r.kernel.org, eilong@...adcom.com, kevin.wells@....com,
	w.sang@...gutronix.de, jeffrey.t.kirsher@...el.com,
	joe@...ches.com, davem@...emloft.net, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4] lpc32xx: Added ethernet driver: smp_wmb()

On Tue, Mar 06, 2012 at 02:03:28PM +0000, Ben Hutchings wrote:
> On Tue, 2012-03-06 at 11:43 +0100, Roland Stigge wrote:
> > On 03/05/2012 11:45 PM, Ben Hutchings wrote:
> > >> +	/* Clear and enable interrupts */
> > >> +	writel(0xFFFF, LPC_ENET_INTCLEAR(pldat->net_base));
> > >> +	lpc_eth_enable_int(pldat->net_base);
> > >> +
> > >> +	/* Get the next TX buffer output index */
> > >> +	pldat->num_used_tx_buffs = 0;
> > >> +	pldat->last_tx_idx =
> > >> +		readl(LPC_ENET_TXCONSUMEINDEX(pldat->net_base));
> > > 
> > > Doesn't this need to be done *before* enabling interrupts?  Also, I
> > > think you need an smp_wmb() so that the interrupt handler is guaranteed
> > > to see all these writes.
> > 
> > Do you mean _one_ smp_wmb() directly after lpc_eth_enable_int() (which
> > I'm moving behind the above code?
> 
> The sequence should be
> 
> 	pldat->state = values...;
> 	smp_wmb();
> 	enable_interrupts();

Is this correct?

"SMP BARRIER PAIRING" and "EXAMPLES OF MEMORY BARRIER SEQUENCES" in
Documentation/memory-barriers.txt suggest that there should be some kind
of pairing with smp_wmb() to ensure correctness.

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